1. Field of the Invention
The invention, in general relates to a method and to an apparatus for defining and at least partially correcting image data and, more particularly, to the definition and correction of data used to form an image on a video or projection screen as well as on other electronic image reproduction devices.
2. The Prior Art
It is often necessary and in some instances desirable to correct or otherwise affect the data used for forming an image on a video or projection screen of the kind used, for instance, in television sets, monitors, displays or projectors. The screen may be part of a cathode ray tube, or other plasma, liquid crystal display or micro reflector technology. Suitably adapted, the invention is also useful in connection with X-ray image forming technology, digital copiers, scanners, image transformers, overhead and other projectors and the like.
The method is practiced to correct errors which typically occur in the reproduction of images. Such errors may be blurs, distortions, irregular illumination, lightness, brightness, hue, color and convergence. Practicing the methods leads do reduced demands on the mechanical, optical and electronic components such as cathode ray guns, LCD panels or projection lenses as well as to a significant adjustment or calibration of such components. However, other shortcomings of projection and image forming screens as well as distortions resulting from oblique projections, known as Keystone distortions, may also be corrected.
The field of use of the invention is by no means restricted to the correction of image data errors, i.e. the reduction or elimination of undesirable changes in image data occurring in a real image reproduction systems as opposed to an ideal system. Basically, the invention may be used in connection with systems in which image data provided for image formation are subjected, prior to their input into the image reproduction system, to modifications definable by predetermining the result of modifying original image data into transformed image data, as, for instance, on the basis of test images.
Hereinafter, “image reproduction system” is intended to refer to such systems in general. Moreover, “digital” and “digitized” image data or “digitized test image data” are not to be understood as image data existing primarily as digital image data, but also as image data prepared by digitizing primarily analog image data.
A video projection apparatus is known from U.S. Pat. No. 5,231,481, for instance, which is provided with a correction feature operating on the basis of feed-back for correcting errors in a projected image caused, for instance, by optical or electrical components of the projection apparatus. Feed-back is provided by a video camera which takes an exposure of a test image projected by the video projection apparatus. The exposure taken and stored by the video camera is compared to image data of the original of the test image which is deemed to be free of errors, for the purpose of calculating and storing corrective signals. These corrective signals are utilized to control the control units of the projection apparatus in order to improve the quality of the projected image.
A method disclosed by U.S. Pat. No. 5,475,447 relates to the automatic correction of errors of convergence and distortion of an image projected onto a rectangularly framed projection surface by a video projector. The method is based of a video projector which is controlled by a video signal generator. A camera takes an exposure of the projection surface, and by means of a signal processor generates a video signal which is stored in an image storage. A computer calculates the coordinates of the four corner points of the rectangular projection surface and determines the position of reference points within the projection surface on the basis of the coordinates. At positions corresponding to the calculated positions of the reference points a signal generator then generates marked or tagged video signals. The video signal controls the video projector which projects an image including the marks. The digital camera stores the image including the marks in an image storage. A corrective signal for the automatic convergence and distortion correction of the camera is derived from any deviation between the positions of the reference points stored in the image storage and the positions of the stored marks.
The video projection apparatus of U.S. Pat. No. 5,091,773 is also provided with a correction feature operating on the basis of negative feed back for correcting, in a projected image, errors which may be caused, for instance, by the optical system. To this end, a test image consisting of light image points arranged as a raster is projected by the projected apparatus on a projection screen. An image of the image on the projection screen is formed on a substantially opaque filtering mask which is transparent only at those image points at which the image of an error-free test image displays light image points. Corrective signals for controlling the video projector are derived from the value of the light intensity which permeating the filtering mask.
A common feature of the known methods is that a corrective signal for improving the quality of the projected image is calculated on the basis of a test image stored by a digital camera for controlling the image generator of the projection system in order to correct errors of distortion or convergence.
The calculation of the corrective values is complex, and it is valid only for the analyzed image point sites. In respect of intermediate positions, the known methods require interpolations. This, too, is complex and provides no more than a mere approximation. Moreover, if is possible to correct only individual errors, primarily those relating to the geometry of an image. Other errors in the transmission characteristics of the reproduction system are not being addressed. The known methods require manipulation of control electronics or mechanical or optical system. In some instances it is even necessary to modify the image or projection screens. Another disadvantage is that the calculation of the corrective values has to be repeated each time an image is reproduced and for each image reproduction system used.
This would seem to be of particular disadvantage where similar image reproduction systems known to be inflicted with substantially identical problems of image formation are used substantially simultaneously.